1. Field of Invention
This invention relates to a surface light source device of side light type employing a light guide plate. The surface light source device of side light type in the present invention is applied to back lighting in a liquid crystal display, for instance.
2. Related Art
A surface light source device of side light type has been conventionally applied to a liquid crystal display, for instance, and illuminates a liquid crystal panel from the back surface. This arrangement is suitable to reduce thickness of the device as a whole.
In the surface light source device of side light type, a rod-shaped light source such as a cold cathode tube is usually employed as a primary light source to be arranged on the lateral side of a light guide plate (a plate-shaped light guide). Illumination light emitted from the primary light source is introduced into the light guide plate through a side end surface of the light guide plate. The introduced illumination light makes a propagation through the light guide plate, and in this process, emission of light from a major surface of the light guide plate occurs toward a liquid crystal panel.
A well-known light guide plate employed in the surface light source device of side light type described above includes a light guide plate of a type having an approximately uniform thickness and a light guide plate of a type showing a tendency to reduce thickness according to distance from the primary light source. Generally, light guide plates of the latter type emit illumination light more efficiently than the former type.
FIG. 7 is an exploded perspective view showing a surface light source device of side light type employing light guide plate of the latter type. Referring to FIG. 7, a surface light source device of side light type 1 has a light scattering guide plate (a light guide plate made of a light scattering guide) 2, a primary light source 3 arranged on the lateral side of the light scattering guide plate, a reflection sheet 4 and a prism sheet 5 functioning as a light control member. The reflection sheet 4, the light scattering guide plate 2 and the prism sheet 5 are laminatedly arranged.
The primary light source 3 has a cold cathode tube (a fluorescent lamp) 6 and a reflection member (a reflector) 7 of a generally semi-circular section arranged around the cold cathode tube. Illumination light impinges on a side end surface of the light scattering guide plate 2 through an opening in the reflector 7. The reflection sheet 4 adopts a sheet-shaped regular reflection member made of metal foil or the like, or a sheet-shaped diffuse reflection member made of a white PET film or the like.
The light scattering guide plate 2 has a wedge-shaped section, and consists of a matrix made of polymethyl methacrylate (PMMA), for instance, and light transmitting fine particles uniformly distributed in the matrix and having deflective index different from that of the matrix.
FIG. 8, including FIGS. 8A and 8B shows a section taken along a line VIII--VIII in FIG. 7. Referring to FIG. 8, illumination light L from the primary light source 3 is introduced into the light scattering guide plate 2 through an incidence surface T serving as one side end surface of the light scattering guide plate 2, and makes a propagation toward an end portion while undergoing repetitive reflection between a plane (which will be hereinafter referred to as "a slope"), along which the reflection sheet 4 is disposed, and a plane (which will be hereinafter referred to as "an emitting surface"), along which the prism sheet 5 is disposed. In the process, the illumination light L undergoes scattering caused by the light transmitting fine particles. If a reflection sheet 4 is made of a diffuse reflection member, the illumination light is affected by an action of diffuse reflection as well.
Every time the illumination light L repeats reflection on the slope, the angle of incidence with respect to the emitting surface is reduced little by little. Reduction in angle of incidence results in an increase of a component equal to the critical angle or less with respect to the emitting surface, urging emission from the emitting surface onward. It is thereby possible to prevent output light from being insufficient in an area distant from the primary light source 3.
Illumination light L1 emitted from the emitting surface undergoes scattering caused by the light transmitting fine particles or diffuse reflection caused by the reflection sheet 4, and therefore, shows the properties of scattered light. However, a main propagation direction of the emitted illumination light L1 is tilted toward an end portion (a direction reverse to the primary light source 3) with respect to the emitting surface. That is, the emitted illumination light L1 has directivity. The surface light source device of side light type 1 employing the above light guide plate generates illumination light having emitting directivity.
The prism sheet 5 is made of a light transmitting sheet material such as polycarbonate and has a prism surface. The prism sheet 5 is arranged so that the prism surface faces the light scattering guide plate 2. The prism surface is composed of a large number of projections, each of which has a triangular section and runs substantially parallel to the incidence surface T of the light scattering guide plate 2. The prism sheet 5 modifies a main emitting direction of the emitted illumination light L1 with a slope of each projection so as to emit the illumination light L1 in a frontal direction of the emitting surface. There is also a case where a prism sheet having prism surfaces on both surfaces is used. In this case, projections forming one prism surface run in a direction respectively orthogonal to projections forming the other prism surface.
In general, the surface light source device of side light type 1 employing the light guide plate of the wedge-shaped section as described above emits illumination light in a frontal direction more efficiently than a surface light source device of side light type employing a light guide plate substantially uniform in thickness.
In some cases, surface light source devices of side light type may employ, as a light guide plate with emitting directivity, a light guide plate having a scattering film, a roughened surface or the like formed on one surface and/or the other surface (i.e. on an emitting surface and/or a back surface) of a transparent member or a semi-transparent member in the shape of a wedge or the like. Such light guide plates also emit illumination light in a frontal direction efficiently.